1. Field of the Invention
The present invention relates to image forming apparatus, and more particularly to an image forming apparatus having an automatic exposure mode and a manual exposure mode.
2. Description of the Related Arts
Presently, copying apparatus for reproducing images of orignals on copy paper are proposed and available which have an automatic exposure mode in which the amount of exposure is automatically determined on detecting the density of the original, and a manual exposure mode in which the operator determines the amount of exposure. Such copying apparatus are adapted to be set in the automatic exposure mode usually in preference. More specifically, the automatic exposure mode is selected automatically immediately after the power supply has been turned on or when the apparatus is not manipulated in any way within a predetermined period of time after operation.
In recent years, multicolor copying apparatus have also been developed which have a monocolor mode wherein images of originals are reproduced in a desired color, and a multicolor mode wherein images of documents are reproduced in the original color. Attempts are also made to provide such multicolor copying apparatus which are settable in the automatic exposure mode and the manual exposure mode described above and which are thereby made more convenient to use and given improved image reproducibility.
However, the monocolor mode and the multicolor mode are different in the requirement to be fulfilled as to the images to be formed. In the monocolor mode, the copy image must be in contrast with the background regardless of the color of the background, whereas in the multicolor mode, the original needs to be reproduced with fidelity. Accordingly, automatic exposure is useful in the monocolor mode, but if automatic exposure is resorted to in the multicolor mode, the exposure voltage is likely to vary depending on the color or density of the original to result in objections. For example, grayish areas will not be reproduced, or areas of a particular color will not always be reproduced with the same color density by being influenced by the surrounding pattern. Consequently, if multicolor copying apparatus are made settable in the automatic exposure mode always in preference, there arises the problem that the multicolor mode is unable to produce proper copy images unless the automatic exposure mode is changed to the manual exposure mode.